1. Field of the Invention
This invention relates to the measurement of oxygen consumption and particularly to the measurement of the comsumption of oxygen in a biological oxidation system.
2. Description of the Problem and the Prior Art
Oxygen is consumed in a biological oxidation system, (e.g. an aerobic culture of microorganisms) by the action of the microorganisms while organic material is oxidized into synthesized cell material and other oxidation products and while synthesized cell material is oxidized further. The amount of oxygen consumed by such a system is of importance in sanitary engineering applications which are concerned, for example, with waste streams (both before and after treatment), effluents from treatment processes, and the quality of natural waters. It is common to express the comsumption of oxygen in a biological oxidation system in terms of the biochemical oxygen demand (BOD), which is the amount of oxygen consumed by the system in a predetermined interval, typically a period of five days. The measurement, however, may require a period as long as twenty days to determine the ultimate amount of oxygen consumed by the system.
It is important to be able to make a fundamental measurement of oxidiation in a biological oxidation system in a much shorter period of time, such as a fraction of an hour. For example, in the treatment of sewage, it is desirable to be able to continuously monitor the oxidation processes in order to determine the degree of completion and progress of oxidation reactions.
It has been found that a fundamental quantity to be measured in any biological oxidation system is the amount of oxygen required to convert organic material in the system to new cell material and other oxidation products. This amount of oxygen, herein, designated energy oxygen is distinct from the oxygen consumed by the system in auto-oxidation, which is the term given to oxidation of previously synthesized cell material. It is generally believed in the literature that the energy oxygen per unit quantity of a particular organic material is constant regardless of the nature of the biological oxidation process. Hence, for a particular material, energy oxygen is the same whether the material is being oxidized slowly or rapidly. Thus a technique for measuring energy oxygen, wherein oxidation is accelerated, provides a basis for making a fundamental measurement of the oxidation process in a relatively short time.
In the past, the measurement of oxygen consumption had generally been limited to the laboratory where direct measurements were made using respiration devices. A disadvantage of such respiration devices is that they provide oxygen consumption data only for a small captive sample contained within the respirometer. Measurements cannot be made of the rate of oxygen consumption in systems outside the respirometer, and hence the respirometers are not suitable for the continuous monitoring of oxidation processes.
In response to the problems mentioned above, Vernon T. Stack, Jr. developed a biological oxygen demand analyzer which is described in detail in U.S. Pat. Nos. 3,510,406 and 3,510,407. The U.S. Pat. 3,510,406 describes an apparatus which includes a pair of dissolved oxygen probes. The system includes what could be called a sludge pot wherein the bacterial growth, sludge, and the liquid sample are mixed. The mixed solution of sludge and sample are then passed through a sealed fluid path, with the dissolved oxygen probes positioned at the inlet and outlet openings of the path. The organic material in the sample combines with the oxygen and bacterial growth in the sludge pot, all of which are passed through the sealed path. The oxygen concentration in the liquid before and after passage through the fluid path is determined by the corresponding dissolved oxygen probes positioned at the inlet and outlet. The reading from the inlet probe is delayed for a time equal to the transit time. The probe readings are then fed to a time integrating device, such as a recorder. The signal indicating the difference between the two DO probe readings gives an indication of the rate of oxygen comsumption.
This system suffers from numerous disadvantages. First of all, since the bacterial growth combines with the sample in the sludge pot, oxygen comsumption occurs not only while the combined liquid mixture passes through the sealed fluid path but also in the sludge pot itself. Thus the oxygen rate as determined from the time recorder chart is not totally accurate. A correction factor has to be introduced to compensate for the additional oxygen consumed in the slude pot itself. Moreover, due to the fact that a pair of oxygen probes are employed and a differential reading between these has to be taken, the difficult task of matching and calibrating the two probes influences the accuracy of the oxygen comsumption rate determination. Further, the storage of the inlet probe reading for the period of time it takes the sample to pass through the sealed path necessitates a certain electronic time delay capacity which makes the equipment additionally complex. Also, in a continuously monitoring system which necessitates the sequential analysis fo liquid samples to detect changing levels of organic material, the time between samples is an important factor. The time between samples is excessively long due to the fact that the decant portion of the cycle, i.e., the liquid bleed-off portion, is delayed excessively while the sludge in the sludge pot settles out to a level below the decant level. Finally, the pumping of the sludge through the system tends to foul up the oxygen probes and the pumping equipment which necessitates mechanical stirrers and brushing apparatus to remove whatever sludge might accumulate on the system hardware.
It is therefore an object of this invention to improve the accuracy of a biological oxidation analyzer system.
It is another object of this invention to increase the number of sample evaluations per unit time in a biological oxidation analyzing system.
It is still another object of this invention to provide a biological oxidation system which will be free of the problems associated with maintaining large amounts of sludge, thereby extending its life and usefulness.
It is yet another object of this invention to make a relatively fast measurement of the oxygen consumption.
It is another object of this invention to determine the energy amount of oxygen consumed by a biological oxidation system.
It is still another object of this invention to provide a relatively less complex and more reliable biological oxidation system.